1. Field of the Invention
The present invention relates to an apparatus for restoring carrier waves and a method therefor in receiving a VSB signal, and more particularly, to an apparatus for restoring carrier waves by selectively controlling an operation of a matching filter which inputs broadcast signals and a method therefor.
2. Description of the Background Art
FIG. 1 is a schematic block diagram of a carrier wave restoring apparatus using an analog signal in accordance with a conventional art.
As shown in the drawing, the carrier wave restoring apparatus using an analog signal processing method includes: a frequency synthesizer 1 for generating a first local oscillating frequency to tune a channel frequency desired by a user; a tuner 2 for receiving a broadcasting signal received by an antenna, turning a frequency of the received broadcasting signal according to the first local oscillating frequency outputted from the frequency synthesizer 1 and outputting a tuned signal of which gain has been controlled by an auto gain control (AGC); a SAW filter for receiving the signal outputted from the tuner 2 and band-pass filtering it; an intermediate frequency amplifier 4 for amplifying the band-pass filtered signal to an intermediate frequency(IF), controlling its gain, and outputting the IF signal as gain-controlled and amplified; a reference frequency oscillator 5 for generating a local oscillation frequency signal having a certain frequency, to generate an xe2x80x98I (In-phase)xe2x80x99 signal and a xe2x80x98Q (Quadrature)xe2x80x99 signal from the amplified IF signal outputted from the IF amplifier 4; a 90xc2x0 shifter 5-1 for shifting a phase of the local oscillation frequency signal outputted from the reference frequency oscillator 5; a multiplier 4-1 for multiplying the IF signal outputted from the IF amplifier 4 and the signal outputted from the 90xc2x0 shifter 5-1, to output an xe2x80x98Ixe2x80x99 signal; a multiplier 4-2 for multiplying the IF signal and the local oscillation frequency signal outputted from the reference frequency oscillator 5, to output a xe2x80x98Qxe2x80x99 signal; an automatic frequency control(AFC) filter 6 for detecting an error component of a carrier wave from the xe2x80x98Ixe2x80x99 signal outputted from the multiplier 4-1; a limiter 7 for receiving and processing the signal outputted from the AFC filter 6 and outputting pulse signals; a multiplier 7-1 for multiplying the pulse signals outputted from the limiter 7 and the xe2x80x98Qxe2x80x99 signal outputted from the multiplier 4-2; an automatic phase control (APC) filter 8 for detecting a frequency and a phase component from the signal outputted from the multiplier 7-1 and compensating the phase component; and a voltage control oscillator (VCO) 9 for receiving the phase compensated signal outputted from the APC filter 8 and outputting a second local oscillation frequency signal to compensate the carrier wave into the tuner 2.
The carrier wave restoring apparatus using an analog signal processing method as shown in FIG. 1, however, is disadvantageous in that when the carrier waves is demodulated in a baseband by using an analog signal processing method, it is difficult to control the phase difference between the xe2x80x98Ixe2x80x99 and xe2x80x98Qxe2x80x99 channels precisely by 90xc2x0, analog circuit elements are easily degraded as they are susceptible to temperature characteristics, and the analog circuit elements are hardly integrated either.
Meanwhile, a quadruple orthogonal amplitude modulation(QOAM) method according to a conventional art, since the gains of the xe2x80x98Ixe2x80x99 signal and the xe2x80x98Qxe2x80x99 signal and the phase difference therebetween should be controlled equivalently, does not perform to restore a carrier wave easily, but includes many steps to do.
Thus, in an effort to solve the problem, a method for demodulating carrier waves in a digital region has been proposed.
FIG. 2 is a schematic block diagram of a carrier wave restoring apparatus using digital signal processing method in accordance with another conventional art.
As shown in the drawing, the carrier wave restoring apparatus includes: a tuner 11 for tuning a frequency corresponding to a channel desired by a user; a SAW filter 12 for receiving a turning signal from the tuner 11 and passing a certain band; a multiplier 12-1 for multiplying the signal outputted from the SAW filter 12 by a local oscillation frequency signal, and outputting an analog signal having a certain frequency; an A/D converter 13 for converting the analog signal outputted from the multiplier 12-1 to a digital signal; an SRC filter 14 for detecting and outputting only a signal existing in a certain band of the digital signal, which matches the frequency of the inputted signal thereinto; a Hilbert 16 for receiving the signal from the SRC filter 14, delaying a phase of 90xc2x0 and outputting a xe2x80x98Qxe2x80x99 signal; a delay 15 for delaying the signal outputted from the SRC filter 14 until the xe2x80x98Qxe2x80x99 signal is outputted from the Hilbert 16, and then outputting an xe2x80x98Ixe2x80x99 signal; a complex multiplier 17 for receiving the xe2x80x98Ixe2x80x99 and xe2x80x98Qxe2x80x99 signals, multiplying them by a carrier wave, and outputting an xe2x80x98Ixe2x80x99 and xe2x80x98Qxe2x80x99 signal of baseband; an infinitive impulse response (IIR) filters 18 and 20 for filtering the xe2x80x98Ixe2x80x99 and xe2x80x98Qxe2x80x99 signals outputted from the complex multiplier 17, respectively; a delay/limiter 19 for delaying the signal outputted from the IIR filter 18 and outputting pulse signals; a multiplier 19-1 for multiplying the pulse signals outputted from the delay/limiter 19 by the signal outputted from the IIR filter 20, and outputting an error component of a frequency and a phase; a loop filter 21 for accumulating the error component outputted from the multiplier 19-1; and a numerically controlled oscillator 22 for receiving the accumulated error component outputted from the loop filter 21, and outputting 0xc2x0 and 90xc2x0 phase signals to the complex multiplier 17, so as for the complex multiplier 17 to compensate the error in the carrier wave.
The differences between the carrier wave restoring apparatus using the analog signal processing method and the carrier wave restoring apparatus using the digital signal processing method are in that, the former compensates the tuning frequency to restore the carrier wave by the voltage control oscillator 9 as used in the latter of FIG. 1 and the latter compensates the frequency by the NCO 22 of FIG. 2.
Of the apparatuses, in the aspect of integrated circuit, the carrier wave restoring apparatus using the digital signal processing method is considered to be better than the carrier wave restoring apparatus using the analog signal processing method.
FIG. 3 is a schematic block diagram of a carrier wave restoring apparatus using a digital signal in accordance with another conventional art, which additionally includes A/D converters for converting signals outputted from the multipliers 4-1 and 4-2 to digital signals and outputting an xe2x80x98Ixe2x80x99 signal and xe2x80x98Qxe2x80x99 signal, respectively, in the carrier wave restoring apparatus using the analog signal processing method of FIG. 1.
The carrier wave restoring apparatus using the digital signal processing method of FIG. 3 separates an IF signal into an xe2x80x98Ixe2x80x99 channel and a xe2x80x98Qxe2x80x99 channel in the analog region, converts them to digital signals by using the two A/D converters 31 and 32, so that the carrier wave is restored by the signals from the VCO 9 after processing the converted signals.
However, the carrier wave restoring apparatuses using the analog and digital signals processing methods still have problems that, in case that the a channel of a frequency is being received is changed or in case that power is supplied to the system at an initial stage (initial mode), a carrier wave deviation occurs, and when the thusly occurred carrier wave passes the matching filter, the carrier wave component is attenuated, which is hardly restored. In addition, the same problem also arises in the case that a carrier wave deviation occurs due to the aging of the analog circuit elements. Especially, the conventional carrier wave restoring apparatuses as described above is not capable of restoring a carrier wave as the carrier wave deviation possibly occurring due to various factors is greater.
Therefore, an object of the present invention is to provide an apparatus for restoring carrier waves accurately and quickly by selectively operating a matching filter which inputs broadcast signal and a method therefor.
Another object of the present invention is to provide an apparatus for restoring carrier waves and a method therefor in which when a broadcast signal is received by a receiver, at an initial stage, the broadcast signal is bypassed through a matching filter of the receiver and restored carrier waves, and when a magnitude of a accumulated sign bit of the carrier wave is greater than that of a predetermined threshold, the matching filter is operated, so that the signal outputted from the matching filter is processed to restore the carrier wave.
To achieve these and other advantages and in accordance with the purpose of the present invention, there is provided an apparatus for restoring carrier waves comprising: a filtering unit for filtering a broadcast signal and outputting a match filtering signal; a multiplier for receiving the matching filtering signal and generating an xe2x80x98Ixe2x80x99 (Inphase) signal and a xe2x80x98Qxe2x80x99 (Quadrature) signal; an error detector for accumulating a sign bit of a DC component of the xe2x80x98Ixe2x80x99 signal, comparing it with a predetermined threshold, and outputting a control signal to the filtering unit; and a gain controlling unit for receiving the xe2x80x98Ixe2x80x99 signal and the xe2x80x98Qxe2x80x99 signal from the multiplier and outputting a gain control signal to control a carrier wave restoration velocity according to the control signal of the error detector.
To achieve these and other advantages and in accordance with the purpose of the present invention, there is provided a method for restoring carrier waves comprising the steps of: match filtering a broadcast signal; generating an xe2x80x98Ixe2x80x99 (Inphase) signal and a xe2x80x98Qxe2x80x99 (Quadrature) signal of the match filtering signal; comparing a magnitude of a sign bit of a DC component of the xe2x80x98Ixe2x80x99 signal with a predetermined threshold, and outputting a control signal; and generating a gain control signal to control a carrier wave restoration velocity according to the control signal.